1. Field of the Invention
This invention relates to a thrust bearing, and more particularly it deals with a tapered-land thrust bearing.
2. Description of the Prior Art
A thrust bearing, particularly a tapered-land thrust bearing, generally has on its bearing surface a plurality of tapered surfaces, flat surfaces and grooves which are arranged in the direction of rotation of a thrust collar positioned against the bearing surface. This type of tapered-land thrust bearing is known as from the publication "BEARING DESIGN AND APPLICATION" published by McGraw-Hill Book Incorporated. The tapered-land thrust bearing has been used in many applications because of its high load bearing capacity.
In the tapered-land thrust bearing of the prior art, there has generally been provided lubricant supply means which includes either peripheral grooves for supplying lubricant formed on the inner side of radial grooves interposed between the flat surfaces and tapered surfaces of the bearing for communicating the radial grooves with one another or apertures formed in the intermediate portions of the radial grooves for supplying lubricant thereto. Also, the radial grooves are each formed on the outer side thereof with throttle means for restricting the flow rate of the lubricant flowing outwardly along the radial grooves.
In this type of tapered-land thrust bearing, a thrust load is borne by a film of lubricant of high pressure formed between the tapered and flat surfaces of the bearing surface and a thrust collar positioned against the bearing surface by rotation of the thrust collar. Owing to the fact that the flow rate of the lubricant flowing outwardly along the radial grooves is restricted by the throttle means as described hereinabove, the lubricant between the thrust collar and the bearing surface has tended to have its temperature increased by rotation of the thrust collar and the lubricant of high temperature has tended to be transferred to the adjacent tapered surface by flowing through each groove. This phenomenon which is generally referred to as hot oil carryover may cause the temperature of the lubricant to rise above the permissible temperature range which is determined by a reduction in the strength of the bearing material, oxidation of the lubricant and other factors. Thus when a thrust bearing is used under high speed and high load conditions, the bearing may sometimes become unfit for use because of seizure.
Meanwhile the aforesaid hot oil carryover also takes place in a pivoted-pad type thrust bearing. A proposal to alleviate the phenomenon is described, for example, in Japanese Patent Publication No. 4130/73 which provides an umbrella sprayer system between the adjacent pads so as to supply cooled lubricant to the leading edge of each pad as well as to scrape off the hot lubricant transferred from the pad of the preceding stage.
When a pivoted-pad type thrust bearing provided with this device is used with a large type rotary machine, such as turbine, compressor, etc., adapted for service under high speed and high load conditions wherein the number of revolutions exceed 15000 rpm and the pressure applied to the bearing surface is as high as 50 kg/cm.sup.2, the following problem would be raised. Normal operation of the bearing would require not only a supply of lubricant and an increase in the pressure of the lubricant supplied but also the provision of a lubricant film of a thickness above the minimum level and the restriction of the maximum bearing temperature. In the pivoted-pad type thrust bearing described hereinabove, the pads receive a supply of cooled lubricant but the pads themselves are not cooled. Thus the temperature of the pads themselves would increase and consequently the temperature of the lubricant film between the pads and the thrust collar would also increase. This would cause a reduction in the viscosity of the lubricant film and the thickness of the lubricant film would be reduced and become unable to cope with the high load condition. From the foregoing, it will be understood that it is impossible to let a thrust bearing operate normally under high speed and high load conditions merely by supplying cooled lubricant to the bearing surface.